WLM is an optional service of AIX and can be started manually or automatically. The wlmcntrl command allows you to start and stop WLM.
All processes existing in the system before WLM is started remain unclassified, and are not monitored by WLM. That is why, in normal system mode, it is best start WLM early in the system initialization process. For instance, WLM could be started by a script run from /etc/inittab.
WLM monitors and regulates the resource consumption at the class level. This means that WLM deals with the sum of the resources used by every process in the class.
WLM attempts to keep active classes close to their targets. Since there are few constraints on the values of the various limits (as mentioned in Setting Up WLM), the sum of any of the limits across all classes could far exceed 100%. In this case, if all of the classes are active, the limit cannot be reached by all classes. WLM regulates the CPU consumption by adjusting the scheduling priorities of the threads in the system according to how the class they belong to is performing, relative to its limits and target. This approach guarantees a CPU consumption averaged on a certain period of time, not the CPU consumption on very short intervals (for example, 10ms ticks).
For example, if class A is the only one active, with a CPU minimum of 0% and a CPU target of 60 shares, then it gets 100% of the CPU. If class B, with a CPU minimum limit of 0% and a CPU target of 40 shares, becomes active, then class A's CPU utilization progressively decreases to 60% and class B's CPU utilization increases from 0 to 40%. The system stabilizes at 60% and 40% CPU utilization, respectively, in a matter of seconds.
This example supposes that there is no memory contention between the classes. Under regular working conditions, the limits you set for CPU and memory are interdependent. For example, a class may be unable to reach its target or even its minimum CPU allocation if the maximum limit on its memory usage is too low compared to its working set. Processes in the class wait to begin.
To help refine the class definition and class limits for a given set of applications, WLM provides a reporting tool, wlmstat, which shows the amount of resource currently being used by each class.